Mirror an object feet away from parent object instead right next to it?
In object mode choose your wall and go into edit mode. Select the front face of your wall and then shifts. Choose Cursor to selected. It will place the 3D cursor at the centre of the face (the front of your wall).



Go back into Object mode and select your door (the pink cube in my case). Right click and select Set Origin > Origin to 3D cursor. 



The pivot point of the cube will move to the centre of your wall (front face).

Add a mirror modifier to the cube and set the axis (in my case the Y)



The mirrored cube will mirror across the pivot point.

For your walls if they are planes (single faces) then make sure your normals are facing the interior of the room and turn off backface culling. This means you can work inside your room without the faces (walls getting in the way).